In the English language, the normal plural of "virus" is "viruses". This form of the plural is correct, and used most frequently, both when referring to a biological virus and when referring to a computer virus. The forms "viri" and "virii" are also used as a plural, although less frequently. There is disagreement over whether these forms should be considered correct.

The plural virii is frequently perceived to be founded on a misunderstanding of Latin plurals such as radii. It may have originated as whimsical usage on BBSs (see also: leet). The virii form is used most frequently, although not exclusively, among crackers and computer virus writers with reference to computer viruses. Most computer professionals unaffiliated with the warez, cracker, and virus writing scenes use the "viruses" form instead of the "virii" form.

The viri form is used less often. It is sometimes used by professionals, and can refer to both biological and computer viruses. To complicate matters further, viri is already used in Latin as the plural of vir, meaning "man" (thus making viri meaning "men").

Plural of virus in Latin

The word virus never had a plural form in Latin. In antiquity the word had not yet acquired its current meaning. It denoted something like toxicity; venom; a poisonous, deleterious, or unpleasant agent or principle; or poison in the abstract or general sense^{vir stem + i ending from the Latin Dictionary and Grammar Aid, by Kevin Cawley, at the University of Notre Dame, verified 2005/02/26.}

1. ^  The first meaning given for this word, a slimy liquid, slime, in the most widely used Latin–English dictionaries is inaccurate; the error has been corrected in the more recent Oxford Latin Dictionary.
2. ^  There is some debate about what the rules of Latin grammar might imply about the formation of a plural. In Latin virus is generally regarded to be a neuter of the second declension, but the word is so rare that there are no recorded plurals. Possibilities include "vira" (in analog with 2nd declension) and "virus" (in analog with 4th declension masculine, although as a neuter noun the plural of virus in the 4th declension would be "virua").
3. ^  To make matters worse, it has been suggested that due to the Latin form of the word, the study of viruses should not be virology (which would be the study of the vir, "man"), but "virulogy". This spelling is extremely uncommon but it is used by a few universities.

References

• The definitive treatise on the plural of “Virus” (mirror) by Tom Christiansen
• Free On-line Dictionary of Computing entry on virus
• The On-line Medical Dictionary entry on virus
• Discussion regarding the plural of virus
• reference.com FAQ: What is the plural of virus?
• "The plural of 'virus' is 'viruses'"

See also

• Virus entry in Wiktionary